Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmasof a mergeable VMA

[Linus Torvalds]

On Sun, 11 Apr 2010, Borislav Petkov wrote:
> 
> > Conversely, if you still see the oops (rather than the watchdog), that 
> > means that we actually have pages that are still marked mapped, and that 
> > despite that mapped state have a stale page->mapping pointer. I actually 
> > find that the more likely case, because otherwise the window is _so_ small 
> > that I don't see how you can hit the oops so reliably.
> 
> Ok, did test with the all 5 patches applied. It oopsed with the same
> trace, see below. Except one kernel/sched.c:3555 warning checking
> spinlock count overflowing, nothing else. :(

Ok, that preempt-count thing is a real problem, but should be unrelated to 
your issues.

Anyway, so this all means that we definitely have lost sight of an 
'anon_vma', even if page->mapping still points to it, and even though the 
page is still mapped.

I'll see if I can come up with a patch to do the same kind of validation 
on page->mapping as on the anon-vma chains themselves.

> I tried to see whether the page->mapping pointer is stale, I dunno,
> maybe there could be something in the register dump which could tell us
> what's happening.

Sadly, you cannot tell by the pointer. A stale pointer still is a 
perfectly fine kernel pointer, it's just that we've long since released 
the anon_vma it used to point to, and now it points to some random other 
data structure.

> So, it really looks like at least that list_head in anon_vma is
> bollocks, or even the whole anon_vma. So if this is correct, it is
> highly likely that the anon_vma is already freed material or not
> initialized at all.

Yes, it's pretty certain it is long free'd, and re-allocated to something 
else.

		Linus
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